How do I kill a process using Vb.NET or C#?

You'll want to use the System.Diagnostics.Process.Kill method. You can obtain the process you want using System.Diagnostics.Proccess.GetProcessesByName.

Examples have already been posted here, but I found that the non-.exe version worked better, so something like:

foreach ( Process p in System.Diagnostics.Process.GetProcessesByName("winword") )
{
    try
    {
        p.Kill();
        p.WaitForExit(); // possibly with a timeout
    }
    catch ( Win32Exception winException )
    {
        // process was terminating or can't be terminated - deal with it
    }
    catch ( InvalidOperationException invalidException )
    {
        // process has already exited - might be able to let this one go
     }
}

You probably don't have to deal with NotSupportedException, which suggests that the process is remote.

Here is an easy example of how to kill all Word Processes.

Process[] procs = Process.GetProcessesByName("winword");

foreach (Process proc in procs)
    proc.Kill();

Killing the Word process outright is possible (see some of the other replies), but outright rude and dangerous: what if the user has important unsaved changes in an open document? Not to mention the stale temporary files this will leave behind...

This is probably as far as you can go in this regard (VB.NET):

    Dim proc = Process.GetProcessesByName("winword")
    For i As Integer = 0 To proc.Count - 1
        proc(i).CloseMainWindow()
    Next i

This will close all open Word windows in an orderly fashion (prompting the user to save his/her work if applicable). Of course, the user can always click 'Cancel' in this scenario, so you should be able to handle this case as well (preferably by putting up a "please close all Word instances, otherwise we can't continue" dialog...)

You can bypass the security concerns, and create a much politer application by simply checking if the Word process is running, and asking the user to close it, then click a 'Continue' button in your app. This is the approach taken by many installers.

private bool isWordRunning() 
{
    return System.Diagnostics.Process.GetProcessesByName("winword").Length > 0;
}

Of course, you can only do this if your app has a GUI